Method for modeling inductive effects on circuit performance

ABSTRACT

A method for testing a partially fabricated wafer is provided that comprises the following steps: providing a device under test (DUT) and three reference oscillators overlying a substrate of the wafer; measuring the frequencies of the reference oscillators as influenced by transistor characteristics, intra structure parasitics, resistive, capacitive and inductive parasitics; and isolating the inductive parasitics by the appropriate comparisons between the reference oscillators.

TECHNICAL FIELD OF INVENTION

The present invention relates to integrated circuit fabrication, andspecifically to structures and methods for electrical testing and/or forprocess monitoring and/or parasitic extraction accuracy monitoringand/or building statistical interconnect models based on siliconmeasurements.

BACKGROUND OF THE INVENTION

A critical part of semiconductor manufacturing and design is the testingof integrated circuits. Before the functionality tests at the end ofprocessing, additional testing for process monitoring is also useful.Testing for process monitoring uses special test structures which arenot part of the integrated circuit's functionality. Process monitoringhelps to detect problems that may cause reliability problems in thefield, helps to optimize the process for maximum yield, and helps tocatch any process deviations before too much work-in-process is wasted.

Typical electrical test structures for in-process testing are longseries-connected chains of similar elements (to test for excessiveresistance or open circuits). The test structure will also include twoor more “probe pads,” which are flat metal surface that electricalconnection can be made to them in the test lab. These test structuresare manufactured at the same time as the functional circuitry, using thesame process steps, but are specially designed to test particularprocess parameters. (For example, to test for specific contactresistance a chain of thousands of series-connected contacts might becreated.) A large variety of test structures are used by processengineers to test various process stages and device portions.

A “wafer” is a flat disk of semiconductor material on which integratedcircuits are made by micro-fabrication techniques. After fabrication iscomplete, the wafer will be separated into rectangular “dice,” each ofwhich is the electronics portion of one integrated circuit. The dicewill be packaged to provide the end-product integrated circuits.Typically one wafer will provide dozens or hundreds of dice.

When the dice are separated, some of the wafer surface between them iswasted. This space is known as “scribelines,” since the dice wereformerly separated by scribing and fracturing; now that diamond sawinghas replaced scribing, these spaces are also known as “saw streets.”

Test structures are placed within the scribelines of the wafer (and inother places), typically once or more per photolithographic field.Typical scribeline test structures are individually connected tocorresponding probe pads, also located in the scribelines. Since thearea within the scribelines is densely occupied, the size and number ofprobe pads is critical. However, probe pad size has not scaled as fastas other process steps, and the space available within the scribelinesis very limited. Placing multiple test structures and probe pads forthose test structures within this limited area becomes difficult in manyprocesses, and limits the testing of the wafer fabrication process.Limits to the testing capability lead to less reliable integratedcircuits.

Scribeline widths are typically less than a tenth of a millimeter. Probepads are typically made just small enough to fit within the scribeline.These dimensions can be expected to change over time, in dependence onwafer fabrication and separation technologies; but the key point is thateach probe pad occupies a very significant fraction of the limitedavailable scribeline area.

Since each probe pad nearly fills the width of a scribeline, the layoutof test structures in the scribelines is often somewhat one-dimensional.That is, a test structure in the scribelines can be allowed to take upnearly the entire width of the scribeline, and extended along thescribeline as far as necessary.

Some space in the corners of the dice themselves is typically alsoallocated for test patterns, but again the available area is limited.Typically several to several tens of test structures can fit into eachcorner of a die.

Some space for test structures is also available along the edge of thewafer, where the grid of square or rectangular dice meets the unusablewidth of the rounded edge. While these spaces are relatively large, theyare far from the important central areas of the wafer. Thus teststructures in the edge-of-wafer corners cannot provide sufficientlyclose monitoring of process variation, including spatial variationacross the wafer.

One basic tool for process monitoring is the use of pilot wafers. Somemanufacturers will start several pilot or dummy wafers for each waferthat will produce actual chips. While some use of pilot wafers willalways be common (e.g. at the head of each lot), every pilot wafer starttakes the place of a wafer full of salable chips. Thus to the extentthat sufficient process monitoring can be done using on-chip test andmonitoring structures, this is greatly preferable to use of pilotwafers. Dummy wafers, on the other hand, are used to ensure thatequipment has stabilized, e.g. when a bulk furnace is being ramped up orwhen a wet processing station has been refilled. Use of such dummywafers is not motivated by process monitoring needs, and hence would notappear to be subject to trade-off against on-wafer test structures.

U.S. Pat. publication 20020047724, entitled “Multi-state test structuresand methods,” describes a selection capability that radically increasesthe number of test structures per probe pad. By adding a test selectorto the test structure, multiple test structures are multiplexed to one(or more) probe pads. Selection of which test structure is to beaccessed from a given probe pad is preferably performed entirely bycontrol of the voltage applied across the probe pads. In one class ofembodiments, the applied voltage directly determines which teststructure will be accessed. In another class of embodiments, modulationof the applied voltage controls sequential logic that selects one ofmultiple test structures for access.

In the past, circuit delay has been due mostly to transistors. Today,the dominant source of delay in circuits such as ASICs andmicroprocessors is metal interconnect. FIG. 1 is a schematic diagram ofa prior art charge-based capacitance measurement (CBCM) structure formeasuring parasitic capacitance. A paper entitled “An On-Chip, AttofaradInterconnect Charge-Based Capacitance Measurement (CBCM) Technique,”Proc. of IEDM 1996, pp. 69–72, discloses an improved test structure forperforming CBCM that included an on-chip signal generator and anentirely new measurement scheme as well. The resolution limit of themethodology is estimated to be 0.01 fF, hence making it more thanadequate for characterizing parasitic interconnect capacitances.

In this paper, a test structure is disclosed that comprises a pair ofNMOS and PMOS transistors connected in a “pseudo” inverter configurationtwo form two inverters 100, 101. Inverter 100 is a reference inverterused to achieve the highest resolution. Inverter 100 is identical toinverter 101 in every manner except that it does not include the targetcapacitance to be characterized. Inverter 101 is connected to the targetdevice that is to be measured. For example, in FIG. 1 the target deviceconsists of level 2 metal line 111 and level 1 metal line 112 that forman intersection at 110. This structure is used to measure the resultinginterconnect capacitance between the two lines. The reference inverteris connected to metal 1 line 113 that is the same configuration as line112, but does not contain the metal 1 to metal 2 overlap capacitancethat is to be measured.

FIG. 2 is a timing diagram illustrating non-overlapping clock signalsused by the prior art CBCM structure of FIG. 1. The V1 and V2 signals ofFIG. 1 consist of two non-overlapping signals shown in FIG. 2. Thesesignals can be either generated off-chip or on chip. The purpose ofthese non-overlapping waveforms is to ensure that only one of the twotransistors in each test structure inverter is conducting current at anygiven time. Thus, short-circuit current from Vdd to ground iseliminated. When the PMOS transistor turns on, it will draw charge fromVdd to charge up the target interconnect capacitance.

This amount of charge will then be subsequently discharged through theNMOS transistor into ground. An ammeter can be placed at the source ofthe PMOSFET (or, alternatively at the source of the NMOSFET) to measurethis charging current. The actual waveform of this charging current isof no consequence; only its DC or average current value needs to bemeasured. DC current can be easily obtained from any modern currentmeter. The difference between the two DC current values in FIG. 1 isused to extract the target interconnect capacitance as shown byequations 1 and 2 below.I−I _(ref) =I _(net)  (1)C=I _(net) /V _(dd) *F  (2)

CBCM can be used in conjunction with simulation at early processdevelopment stages to provide designers with accurate parasiticinterconnect capacitances; including metal to substrate, interwire, andinterlayer capacitances, as discussed in a paper entitled “Investigationof Interconnect Capacitance using Charge-Based Capacitance Measurement(CBCM) Technique and 3-Dimensional Simulation”, IEEE JSSC, pp. 449–453,March 1998. However, this paper teaches that several test structuresmust be utilized to decouple vertical and horizontal capacitivecomponents from the total capacitance.

Therefore, there is still a need in the art for a way to increase theefficiency of space usage for test structures within the scribelines ofan integrated circuit process.

SUMMARY OF THE INVENTION

An apparatus is provided for testing a partially fabricated wafer usinga test structure located within the scribe lines of the wafer. The teststructure includes a first structure overlying a substrate and coupledto a first probe pad, such that the first structure has a firstparasitic capacitance relative to the substrate. A second structure islocated in proximity to the first structure, such that the firststructure has a second parasitic capacitance relative to the secondstructure. A bias circuit is coupled to the second structure and has aninput coupled to a second probe pad. The bias circuit is operable tobias the second structure in response to a select signal impressed onthe second probe pad. A test signal impressed on the first probe pad isoperable to provide a measure of the first parasitic capacitance whenthe select signal has a first value; and the test signal is operable toprovide a measure of the second parasitic capacitance when the selectsignal has a second value.

A method for testing a partially fabricated wafer is provided thatcomprises the following steps:

a) providing a device under test (DUT) overlying a substrate of thewafer;

b) biasing a second structure located in proximity to the DUT to have afirst electrical state such that a first equivalent test structure isformed;

c) determining a first parasitic parameter associated with the firstequivalent test structure by applying a signal to the DUT while thesecond structure is in the first electrical state and measuring aresponse that is indicative of the first parameter;

d) biasing the second structure to have a second electrical state suchthat a second equivalent test structure is formed; and

e) determining a second parasitic parameter associated with the secondequivalent test structure by applying a signal to the DUT while thesecond structure is in the second electrical state and measuring aresponse that is indicative of the second parameter.

Advantages of the disclosed methods and structures, in variousembodiments, can include one or more of the following: more teststructures can be used on a given wafer; test structures can be madelarger; quicker correction of process deviations; increased efficiencyof use of wafer area; increased efficiency of use of scribeline area;fewer probe pads are needed; increased yield; and increased capabilityfor “early warning” testing increases reliability of the integratedcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention, wherein:

FIG. 1 is a schematic diagram of a prior art charge-based capacitancemeasurement (CBCM) structure for measuring parasitic capacitance;

FIG. 2 is a timing diagram illustrating non-overlapping clock signalsused by the prior art CBCM structure;

FIG. 3 is an illustration of a partially completed wafer showing theplacement of individual chips and scribelines in which are located teststructures according to the present invention;

FIG. 4 is a schematic illustrating an embodiment of a CBCM teststructure according to aspects of the present invention;

FIG. 5 is a pictorial representation of various parasitic capacitancesthat exists in the structure of FIG. 4;

FIG. 6 is a flow chart of a test method using the structure of FIG. 4;

FIG. 7 is a pictorial of a more complex structure illustrating variousparasitic capacitances that can be isolated using a single structure,according to aspects of the present invention;

FIG. 8 is a schematic illustrating use of a common reference circuit forseveral CBCM measurement circuits;

FIG. 9 is a schematic illustrating the use of demultiplexing circuits toexpand the number of DUTs;

FIG. 10 is a schematic of a reference ring oscillator;

FIG. 11 shows a schematic of a reference ring oscillator employed inmeasuring the RC parasitics; and

FIG. 12 is a schematic of a ring oscillator influenced by inductanceparasitics.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. More over, some statements may applyto some inventive features but not to others.

FIG. 3 shows a wafer 300 on which individual chips 302 are being built.The chips 302 are divided by scribelines 304. As described below, thearea of the scribelines 304 is used to place test structures 310 andtest selectors 320, so that area on the wafer is used efficiently. Thetest selectors 320 comprise circuit elements that accept a select signaland multiplex one of several biasing signals onto the test structuresdepending on that input signal. In this way, a single probe pad can beused to effectively vary a test structure 310 on a wafer, thus reducingthe number of structures needed to determine parasitic values associatedwith a three dimensional structure and thereby improving the efficientlyof the testing process, as will now be described in more detail.

The test structures and test selectors are made on the wafer surface inthe same way that other circuit features are produced. In the preferredembodiment, this includes the typical integrated circuit fabricationprocesses, such as photolithography, etches, depositions, etc. Thesefeatures are placed on the wafer area that is later used for scribelinesthat separate the several chips on the wafer. When the chips arephysically separated from one another (by sawing, breaking, etc.), thetest structures and test selectors are destroyed. These test structuresand test selectors are normally not connected to any circuitry on thechips themselves.

FIG. 4 is a schematic illustrating an embodiment of a CBCM teststructure according to aspects of the present invention. The overalltechnique is similar to prior art structures; however, in thisembodiment reference inverter 400 is located separately from the deviceunder test (DUT) inverter 401. In order to achieve the highestresolution, a null line segment 413 is included with the referenceinverter that is identical to null line segment 412. Null line segment412 is a necessary connection line to the DUT line 410, but it'sparasitic capacitance is not to be included as part of DUT line 410. Anadvantage of locating reference inverter separately from DUT inverter401 will be discussed with reference to FIG. 8. In another embodiment ofthe present invention, reference inverter 400 may be co-located with DUTinverter 401.

DUT line 410 extends over the substrate of the wafer for some distanceand therefore has a parasitic capacitance to the substrate. A secondstructure 411 is located in proximity to DUT line 410 and is fashionedas side by side tracks on either side of line 410. In another teststructure, other configurations may be employed, as will be describedlater. Second structure 411 also has parasitic capacitive effects withDUT line 410. DUT line 410 and structure 411 are representative ofparallel signal lines used for interconnect in a finished circuit. Theline to substrate and line-to-line parasitic values are an importantfactor in the design of the finished circuit and therefore need to bemonitored during fabrication of a wafer.

In order to test both parameters with only a single structure, selectcircuitry 420 is provided along with another inverter 402. Inverter 402will be referred to as a bias inverter. Reference inverter 400 and teststructure inverter 401 identical in every manner. Bias inverter 402 islikewise similar to inverter 401, but may be modified slightly in orderto maintain similar signal characteristics between line 410 andstructure 411. Selector 420 is controlled by select signal 421 andcauses different bias conditions to be selected and applied to structure411.

By biasing structure 411 to a steady voltage, such as to substratepotential, the combined parasitic capacitance of DUT line 410 to thesubstrate and also to structure 411 can be measured. Alternatively,another steady voltage, such as Vdd could be applied to structure 411 asa bias. In this embodiment, when select is de-asserted (a logical lowsignal) each multiplexer of select circuit 420 passes a logical onesignal to the two gate inputs of bias inverter 402. This causes the PMOStransistor to turn off and the NMOS transistor to connect structure 411to ground, thereby biasing the structure with a steady ground potential.The CBCM measurement is then made by applying a periodic voltage to theDUT line 410 from test inverter 401 in response to non-overlapping clocksignals pclk and nclk. Test voltage V_(test) is applied to a probe padconnected to inverter 401 for this purpose and a resulting DUT currentflow I_(DUT) is measured that is representative of the parasiticcapacitance value. Similarly, a null current I_(null) is measured byapplying reference voltage V_(ref) to a probe pad connected to nullinverter 400. Null inverter 400 also is controlled by non-overlappingclock signals pclk and nclk. The parasitic capacitance is thendetermined using equation 3.C=(I _(DUT) −I _(NULL))/V _(dd) *F  (3)

Then, by asserting the select signal (a logical one), each multiplexerof select circuit 420 passes the non-overlapping clock signals pclk,nclk to the two gate inputs of bias inverter 402. This then causesstructure 411 to be biased with a periodic voltage that has the samewaveform as the periodic voltage applied to DUT line 410. Because ofthis, no charge will flow between DUT line 410 and structure 411 due toparasitic capacitance. Therefore, the test structure is effectivelychanged to form a virtual test structure that only comprises the DUT.Thus, when the CBCM measurement process is again performed, as describedabove, only the parasitic capacitance of DUT line 410 with reference tothe substrate will be measured.

Thus, by placing two different bias signals on structure 411, twodifferent virtual test structures are formed and two different parasiticcapacitance parameters can be measured using only one physical teststructure.

FIG. 5 is a pictorial representation of various parasitic capacitancesthat exists in the structure of FIG. 4. Capacitance 500 represents aparasitic capacitance between DUT 410 and substrate 510 due to thephysical proximity of DUT 410 to substrate 500. Of course, it is to beunderstood that capacitance 500 is a distributed capacitance along thelength of DUT 410, but is shown for simplicity as a single element.After capacitive measurements are taken using the CBCM technique,generally the measured value is normalized to a capacitance/length valuebased on the length of DUT 410.

Parasitic capacitance 501 and 502 likewise represent distributedparasitic capacitance that occur between DUT 410 and secondary structure411 represented by 411 a, 411 b due to the proximity of structure 411 toDUT 410.

By applying a first constant voltage bias signal to structure 411 viasignal line 411.1, a CBCM measurement of current drawn into DUT 410 willbe indicative of combined capacitance 500, 501 and 502. Advantageously,by applying a periodic bias voltage to structure 411 via 411.1 thatmirrors the periodic CBCM voltage applied to DUT 410, a virtualstructure is formed that effectively does not include secondarystructure 411. CBCM measurement of current drawn into DUT 410 will beindicative of only capacitance 500, since no charge will flow betweenDUT 410 and structure 411. Thus, capacitance 501 and 502 can be deducedby subtracting the measured value of capacitance 500 from the totalmeasured value of combined capacitance 500, 501, 502. In thisembodiment, element 411 a and 411 b are identical and equally spacedfrom DUT, therefore it can further be deduced that capacitance 501 isapproximately equal to capacitance 502.

FIG. 6 is a flow chart of a test method using the structure of FIG. 4.In step 600, a device under test (DUT) is provided that overlays asubstrate of the wafer. A second structure located in proximity to theDUT is also provided. As discussed above, the DUT has a parasiticparameter relative to the substrate and a parasitic parameter relativeto the second structure.

In step 602, a first virtual test structure is formed by biasing thesecond structure to have a first electrical state. This is done byconnecting the second structure to the substrate, for example

In step 604, a first parasitic parameter is determined by applying asignal to the DUT while the second structure is in the first electricalstate and measuring a response that is indicative of the firstparameter. Using the CBCM technique, measuring the current drawn intothe DUT will provide a current value that is indicative of the combinedcapacitance of the DUT relative to the substrate and relative to thesecond structure.

In step 606, a second virtual test structure is formed by biasing thesecond structure to have a second electrical state. This is done byconnecting the second structure to periodic voltage that is the same orsimilar to a periodic voltage applied to the DUT, for example.

In step 608, a second parasitic parameter is determined by applying asignal to the DUT while the second structure is in the second electricalstate and measuring a response that is indicative of the secondparameter. Using the CBCM technique, measuring the current drawn intothe DUT will now provide a current value that is indicative of only thecapacitance of the DUT relative to the substrate since the secondstructure has been virtually removed from the test structure by theperiodic bias signal.

FIG. 7 is a pictorial of a more complex structure illustrating variousparasitic capacitances that can be isolated using a single structure,according to aspects of the present invention. Element 700 is the DUTand overlays a substrate, not shown for clarity. Elements 701 and 702proximally located in parallel to DUT 700 on a same level. Element 704is proximally located in parallel to DUT 700 on a lower level. Elements703 and 705 are proximally located in a perpendicular arrangement ondifferent levels. Various parasitic capacitances 711–715 exist betweenDUT 700 and elements 701–705 due to the close physical spacing.Advantageously, by connecting various combinations of bias voltages toelements 701–705 via respective terminals 701.1–705.5 various virtualstructures can be formed. By performing a CBCM measurement on eachvirtual structure, the various parasitic capacitances 711–715 can bedirectly measured or deduced, as described previously.

FIG. 8 is a schematic of a test structure 750 that includes many DUTinverters 701.1–701.n and illustrating use of a common reference circuit700. Referring again to FIG. 4, it was mentioned that reference circuit400 optionally could be located separately from DUT inverter 401. Theinventor of the present invention has discovered that a single referenceinverter 700 can be used to improve the resolution of a number of DUTinverters 701.1–701.n, where each DUT inverter 701.x is similar to DUTinverter 401 and reference inverter 700 is similar to reference inverter400. Non-overlapping clock signals nclk and pclk are routed to eachinverter and are provided by dedicated probe pads. Each of DUT inverters701.1–701.n is designed to be identical. Within a local region thatincludes all of DUT inverters 701.1–701.n, fabrication repeatability isgenerally good enough to insure that all of the DUT inverters will havethe same electrical characteristics so that a single reference inverteris sufficient. Advantageously, this allows a smaller overall footprintfor test structure 750.

Another aspect of test structure 750 is that it is divided into banks,as represented by bank 751 that each contain three or four DUTinverters, for example. In this embodiment, bank 750 has three DUTinverters 701.1–701.3 that each have a respective probe pad 703.1–703.3for providing a test voltage V_(test). Bias circuitry that includesselection circuitry 720.1 and bias inverter 702.1 provide a bias voltageto each of the three secondary structures associated with each ofrespective DUTs 710.1–710.3. Advantageously, a single select signalapplied to probe pad 721 controls selection circuitry 720.1 and therebycontrols the bias of each of the secondary structures. Note that each ofthe secondary structures may have a different physical configuration, orthey may all be the same.

Additional banks each have another bias circuit, such a selectioncircuit 720.2 and bias inverter 702.2. All of the selection circuitsreceive a common select signal from probe 721. However, in an alternateembodiment, separate select signals may be provided on separate probepads. This may be useful in a complex structure such as that illustratedin FIG. 7.

A representative test structure 750 may contain 20 probe pads. Two areconnected to receive the non-overlapping clock signals. One is connectedto receive the select signal. Another one is connected to provideV_(bias) to each of the bias inverters 702.x. Another is connected toprovide V_(ref) to the single reference inverter 700. The remainingfifteen probe pads are connected to provide Vtest to each of fifteen DUTinverters 701.x that are organized into five banks. The twenty probepads are arranged in a linear manner and the entire test structure fitswithin a scribeline region. Larger test structures can be configured ina similar manner, for example having forty probe pads.

FIG. 9 shows a test structure wherein a plurality of DUT's 901 through904 are connected to demultiplexer 905. Control lines 906 and 907 may beused to select the DUT to be tested. The depth of the demultiplexerchain may be extended to increase the number of DUTs that may beselected for testing.

In another embodiment of the invention, multiple ring oscillators may beused both as a DUT and as a reference, and the circuit's inductance maybe derived by measuring the frequency of the oscillators.

FIG. 10 shows a ring oscillator 1001 comprised of an even number ofinverters 1002, and a nand gate 1003 interconnected as shown. Thiscircuit, under the control of line 1004 will oscillate at a frequencyF_(r). This frequency will depend on the transistor characteristics andon the intra cell parasitics such as illustrated in FIGS. 5 and 7 arerepresented schematically as 1005. The frequency F_(r) can be measuredconventionally by a counter controlled by a trigger signal.

FIG. 11 shows the ring oscillator 1001 of drawing 10, but in this casethe interconnect between two of the inverters is an external DUTstructure 1101. This structure is surrounded by parallel conductors 1102and 1103 which are connected to a dedicated ground. The frequency F_(RC)of ring oscillator 1001 in this case is controlled by the transistorcharacteristics, the intra cell parasitics, and by the resistance (R)and capacitance (C) parasitics of the DUT structure 1101.

FIG. 12 shows the ring oscillator 1001 of drawing 10. In this case, theDUT structure 1201 is surrounded by actual “aggressor” circuitry 1203and 1204, and the frequency of oscillation FRLCM will be controlled bythe transistor characteristics, the intra cell parasitics, theresistance (R), capacitance (C), self inductance (L), and the mutualinductance (M) parasitics of the DUT structure 1201.

Since the three ring oscillators let us eliminate the effects oftransistor characteristics, intra cell parasitics and RC parasitics ofthe DUT, the inductance parasitics may be easily calculated.

As used herein, the terms “applied,” “connected,” and “connection” meanelectrically connected, including where additional elements may be inthe electrical connection path. “Associated” means a controllingrelationship, such as a memory resource that is controlled by anassociated port. The terms assert, assertion, de-assert, de-assertion,negate and negation are used to avoid confusion when dealing with amixture of active high and active low signals. Assert and assertion areused to indicate that a signal is rendered active, or logically true.De-assert, de-assertion, negate, and negation are used to indicate thata signal is rendered inactive, or logically false.

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the invention will beapparent to persons skilled in the art upon reference to thisdescription. For example, in another embodiment, the non-overlappingclock signals are generated by a clock circuit within the teststructure, rather than being supplied via a probe pad. In anotherembodiment, a single clock is provided via a probe pad that is thenconverted into two non-overlapping clock signals by a circuit includedwith the test structure.

Various types to DUT structures can be used; for example, activecomponents such as transistors can be included in order to measureparasitic parameters of the transistors.

An operational circuit, such as a ring oscillator can be a DUT and anoperation parameter such as frequency can be measured. The select signalcan be used to bias a secondary structure proximate to the oscillatorand a resulting frequency can be measured to determine sensitivity ofthe oscillator to the secondary structure.

In another embodiment, the select signal may be used to select more thantwo bias parameters by using a multi-bit select signal, or by encoding aselect number using a pulse code or various signal levels, for example.

In another embodiment, one bias circuit may be used to bias all of thesecondary structures in a given test structure. Alternatively, oneselection circuit having multiple outputs may be connected to severaldifferent bias inverters so that a single select signal can causedifferent bias conditions to be placed on various secondary structures.In this case, the select signal may be a multi-bit select signal, or maybe encoded by using a pulse code or various signal levels, for example.

In another embodiment, additional multiplexers or demultiplexers may beprovided and used to select from multiple test structures according toan input signal in order to reduce the needed number of probe pads. Whena particular DUT test structure is selected, that test structure isconnected to the probe pads and its electrical characteristics aretested by further selecting various bias values to configure variouseffective structures, as described above.

Advantageously, the test methods described herein may be used formeasuring capacitance of a variety of structures. For example, themethod can be used to monitor memory bit-line and word-linecapacitances. Alternatively, the method can be used to measuretransistor intrinsic capacitance. Alternatively, the method can be usedto separate out capacitances due to contacts and vias in integratedcircuits. Many other types of structures may be measured using themethods described herein as is readily apparent to one skilled in theart of semiconductor circuit design and/or fabrication.

1. A partially fabricated wafer having a test structure overlying thesubstrate of the wafer, the test structure comprising: a first ringoscillator comprising an even number of inverters and a NAND gate havingtwo inputs and an output, said inverters coupled in series having afirst input connected to said output of said NAND gate and a finaloutput connected to said first input of said NAND gate, whereby saidfirst ring oscillator has a frequency of oscillation influenced bytransistor characteristics and intra device parasitics; a second ring anoscillator comprising an even number of inverters, a NAND gate havingtwo inputs and an output, a device under test (DUT) line and a passiveconductor parallel to said DUT line, said inverters coupled in serieshaving a first input connected to said output of said NAND gate and afinal output connected to said first input of said NAND gate and saidDUT line coupled between said output of an inverter and said input of asuccessive inverter, whereby said second ring oscillator has a frequencyof oscillation being influenced by the transistor characteristics, intradevice parasitics, resistive and capacitive parasitics of said DUT linerelative to said passive conductor; and a third ring oscillatorcomprising an even number of inverters, a NAND gate having two inputsand an output, a device under test (DUT) line and an aggressor conductorparallel to said DUT line, said inverters coupled in series having afirst input connected to said output of said NAND gate and a finaloutput connected to said first input of said NAND gate and said DUT linecoupled between said output of an inverter and said input of asuccessive inverter, whereby said third ring oscillator has a frequencyof oscillation being influenced by the transistor characteristics, intradevice parasitics, resistive, capacitive and inductive parasitics ofsaid DUT line relative to said aggressor conductor.
 2. The partiallyfabricated wafer of claim 1, wherein said first, second and third ringoscillators are located in a scribeline of the wafer.
 3. A method fortesting a partially fabricated wafer, comprising the steps of: providinga first ring oscillator comprising an even number of inverters and aNAND gate having two inputs and an output, said inverters coupled inseries having a first input connected to said output of said NAND gateand a final output connected to said first input of said NAND gate;measuring a frequency of operation of the first ring oscillator;providing a second ring an oscillator comprising an even number ofinverters, a NAND gate having two inputs and an output, a device undertest (DUT) line and a passive conductor parallel to said DUT line, saidinverters coupled in series having a first input connected to saidoutput of said NAND gate and a final output connected to said firstinput of said NAND gate and said DUT line coupled between said output ofan inverter and said input of a successive inverter; measuring afrequency of operation of the second ring oscillator while connectingthe passive conductor to a fixed voltage; providing a third ringoscillator comprising an even number of inverters, a NAND gate havingtwo inputs and an output, a device under test (DUT) line and anaggressor conductor parallel to said DUT line, said inverters coupled inseries having a first input connected to said output of said NAND gateand a final output connected to said first input of said NAND gate andsaid DUT line coupled between said output of an inverter and said inputof a successive inverter; and measuring a frequency of operation of thethird ring oscillator while connecting the aggressor conductor to aoscillating voltage.